"Hyperthyroidism in Cats

"Hyperthyroidism in Cats

Health & Safety

Hyperthyroidism is a condition that can affect any breed of cat, male or female. It's a very common glandular disorder where there's too much of a thyroid hormone known as T4 found in the cat's bloodstream. This is a worrying condition that can cause owners a lot of concern although it's usually only seen in older cats. The symptoms become apparent when cats are over 10 years old, with the average age being between 12 and 13 years old.

Symptoms You Need To Watch Out For

The first sign there could be something wrong is when your cat suddenly starts to lose weight and yet their appetite increases. These two factors are the most commonly seen clinical signs of hyperthyroidism, with weight loss seen in around 98% of cats suffering from the condition. Other typical symptoms are as follows:

  • Excessive drinking and thirst
  • Increased need to urinate
  • Hyperactivity
  • Unkempt, messy appearance
  • Panting
  • Diarrhea
  • An increase in shedding
  • Heart disease
  • Increased respiration rate
  • High blood pressure

Sometimes a cat will vomit but this is one symptom that's only really seen in around 50% of cat suffering from hyperthyroidism. The symptoms are a result of the the increased, and excessive amount of T4 in the bloodstream which affects various organs in the cat's body.

How Vets Diagnose Hyperthyroidism

There are quite a few other common diseases seen in older cats that share similar clinical signs of hyperthyroidism – this includes inflammatory bowel disease, diabetes, intestinal cancer and chronic kidney failure. As such vets need to do a variety of tests including blood tests and urinalysis tests to eliminate these diseases and make a correct diagnosis. However, the blood test alone should show that hyperthyroidism could be the cause of the symptoms, due to the elevated T4 levels present in the blood.

The problem with the blood test is there is a small percentage of cats that have normal levels of T4 in their bloodstreams even when they are suffering from the condition. This is often thought to be because another illness is suppressing the levels of T4 in the blood making the disease harder for vets to diagnose – hence they need to carry out more than just a simple blood test.

Hyperthyroidism Treatments

There are several treatments available for hyperthyroidism and each offers some advantages as well as a few disadvantages when treating the condition. One of the main treatments vets like to use is an oral medication which has proved to be a very effective way of correcting the condition within two to three weeks, with most cats recovering extremely well. However, there are some disadvantages with around 20% of cats being affected by the following side effects:

  • Loss of appetite
  • Vomiting
  • Lethargy
  • Blood clotting problems
  • Jaundice
  • Itching around the head and face
  • Blood cell abnormalities – although this particular side effect is pretty rare

However, most of these side effects are pretty mild and do eventually go away with very few cats having to be taken off the medication. Once the condition has been diagnosed, cats need to be medicated for the rest of their lives on a daily basis and their T4 levels need to be regularly checked by a vet too.

Surgery is Another Treatment Option

Another treatment involves surgically removing the thyroid gland. Hyperthyroidism is often caused due to a benign (non cancerous) tumour which is known as a thyroid adenoma found in one or both glands. These can be easily removed but the problem is that older cats don't react well to anaesthetic and often their hearts and other organs are too weak which makes the surgery not a viable option for them.

Radioactive Iodine Therapy

The other treatment for hyperthyroidism is radioactive iodine therapy which is considered as the most effective treatment available. The treatment is given in injection form, and subcutaneously with the solution destroying the hyperfunctioning tissue found in thyroid glands. The beauty of this treatment is there is no need for any surgery or anaesthetic with one single injection achieving the desired results and therefore a cure.

However, the cat treated would have to remain in the veterinary clinic for a period of anything from 10 to 14 days so the level of radioactivity in their urine and faeces can be monitored. When the levels are low enough, the cat will be allowed home. This makes it an expensive treatment but on the upside, it is a very effective way of treating hyperthyroidism.

What Causes Hyperthyroidism?

It's not known exactly what causes the condition but it is thought these could include the following:

  • Iodine levels in certain cat foods – this can be either too high or too low
  • PBDEs which are fire retardant chemicals found in high levels of concentration in certain fish as well as house dust – some cat nutritionists recommend not feeding too much fish based cat food and diets to cats for this reason
  • Soy is known to affect the function of thyroid glands and with many cheaper brands of cat food using soy in their ingredients, it's thought this could be a link to cats contracting the condition. With this said, some top brands of cat food manufacturers use soy in their ingredients so it's always worth checking the label of the food you buy for your cat


If you think your cat has lost a lot of condition and yet their appetites are better than usual, and especially if they are in their senior years, it is worth making an appointment with your vet so they can carry out an initial blood test to see if the levels of T4 are very elevated. The sooner your cat is treated, the more comfortable they will be and you'll have the peace of mind of knowing just what's wrong with your feline friend and how you can help them through their illness.

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